In wire-free baseband applications such as UMTS, WLAN as well as digital television and Bluetooth, digital data is converted to analog signals by means of a digital/analog converter (DAC). The analog signals are then up-mixed to the carrier frequency, are post-filtered and are emitted by means of the power amplifier via the antenna to the air interface. In the baseband applications that have been mentioned, broadband modulation methods such as OFDM, DSSS or other broadband modulation methods are used. These modulation methods make better use of the channel bandwidth and thus achieve higher data rates. The linearity, the phase response and attenuation response of the DAC output are subject to stringent requirements resulting from the broadband modulation methods.
FIG. 1 shows a 9-bit current source DAC with 512 individual current sources. Each current source has a switch, which receives a binary signal from a decoder. This signal determines whether a current source is connected to the node Vout,p or to the node Vout,n. If the digital word is altered by an LSB (Least Significant Bit), a current source is switched from Vout,p to Vout,n. Depending on the bit number, the DAC is fully segmented or partially segmented. If the digital word (000000000) is applied to the DAC, 512 current sources are connected to Vout,n, and no current sources are connected to Vout,p. The digital word (111111111) is applied when fully driven, with all of the current sources being connected to Vout,p and none to Vout,n. As can be seen from FIG. 1, the output currents are converted across the resistors to a differential output voltage Voutdiff=Vout,p−Vout,n.
Various circuit arrangements by means of which the two DAC output currents can be converted to a differential output voltage are known from the prior art. The simplest option is to convert the output currents from the DAC directly to two output voltages across the two resistors illustrated in FIG. 1. This solution is highly linear and does not produce any additional harmonics. The phase and attenuation response are influenced only by that pole which is produced across the resistors with the load capacitances. Another known circuit arrangement amplifies the difference voltage across the DAC output resistors using an inverting differential amplifier circuit. It has been found that these known circuit arrangements do not satisfactorily ensure one major aim, specifically the maintenance of the phase and attenuation profile. For this purpose, it is necessary for the converter to have a high gain/bandwidth product, and for no additional poles and null points to be introduced into the current/voltage transfer function during conversion of the DAC output current to the output voltage. The known circuits arrangements do not have these characteristics to an adequate extent.